1. Technical Field of the Invention
The present invention relates generally to circuits which produce resistance or voltage potential and, more particularly, to integrated circuits which provide a selectable resistance value or voltage potential between two external terminals thereof.
2. Description of Related Art
Traditional potentiometers are mechanical devices whose resistance varies according to a selected physical position of a wiper, which can produce selected electrical potentials. Typically, wiper location is controlled by rotating or sliding the wiper to the desired position. The increased use of integrated circuits and other semiconductor devices includes efforts to develop solid state potentiometers. For example, Xicor's x9MME, discussed in U.S. Pat. No. 5,243,535, is a resistor array composed of 99 resistive elements wherein wiper position is digitally controllable. Another example is provided in Banezhad and Gregorian, "A Programmable Gain/Loss Circuit," 22 IEEE Journal of Solid-State Circuits 1082 (1987), wherein a programmable circuit is disclosed which has gain/loss values ranging from -25.5 dB to +25.5 dB in 0.1 dB increments.
One notable aspect and potential shortcoming of the design of prior art potentiometers is that they utilize conventional integrated circuit manufacturing techniques. This method has several problems which will be apparent from the discussion below. With respect to resistor arrays, it is known that resistance matching between two discrete devices is best accomplished, at the design stage, by designing and building cells of resistors having resistors with equal values and equal numbers of resistors. For example, Maher, et al., U.S. Pat. No. 4,514,703, discloses a resistor chain or attenuator wherein the resistors are substantially similar. In order to obtain a target resistance the interconnections between cells are varied to create the target resistance.
Birkner, U.S. Pat. No. 4,338,195 addresses the precision problem of semiconductor devices by using a programmable resistor array, through electrical fusion, to provide a specified resistance. Birkner does not, however, address the problems of providing precise resistive steps in the digital potentiometer or even recognize the problem of imprecise values of resistance being provided.
A desired resistance of a resistance cell of a digital potentiometer of the prior art, within acceptable tolerances, is only obtained through random good fortune, especially for lower values of resistance. This is especially true when conventional IC fabrication techniques are used. Ordinarily, the resulting resistance is only within a variable tolerance range of the desired resistance. For example, in prior art devices a typical resistance tolerance can be in the range of .+-.15% but match to each other with much tighter tolerances, as high as fractions of a percent. Thus, the traditional design approaches to developing these resistor banks and/or digital potentiometers lend themselves to repetitively yielding high levels of precision only on exactly repeated resistances. However, it is generally known that even the smallest dB steps in attenuators having logarithmic increments do not provide the required resolution to mimic a smoothly changing resistance needed to produce nonlinear steps. Attenuators of this type cause gain distortions that are marginally tolerable at audio frequencies. These distortions however become unsuitable when precise resistances are required, especially at low values of resistance such as on the order of below 100.OMEGA.. The prior art, does not, therefore, appear to provide a solution for a digital potentiometer requiring precise, small, logarithmic steps. This is especially problematical in high-end audio equipment that requires precise tolerance and repeatability.
In addition to not providing precise resistance values, digital potentiometers have other short comings. First, most of the prior art devices at their front end, do not have the capability of directly connecting the control to a switch on a panel, for example of a stereo. While mechanical potentiometers are generally mounted directly to a stereo panel, it is common to interface a digital potentiometer to a switch through a microprocessor which may be under software control. Additionally, when a volume increase is selected, digital potentiometers frequently introduce detectable noise into the signal as different wiper points are selected while a given signal is being amplified.
Finally, the circuits or mechanisms used to switch in and out of the wiper points of a digital potentiometer generally consume large amounts of current and thus generate heat and decrease the mean time between failures of a system using such devices. Thus, what is needed is digital potentiometers which may be directly attached to a switch, as a mechanical potentiometer can be so attached, potentiometers which reduce the noise added to a signal during changes in amplification, potentiometer which provide accurate and predictable attenuation in logarithmic increments to account for human hearing sensitivity, and which consume less power to operate and therefore which are more reliable.